If DS is 802.11b,
then get a router that is b, g, or both. G and B operate on 2.4 ghz frequency. G is merely a faster version of B (54 mbs instead of 11 mbs), so they are interchangeable. A G device coudln't work on a B-only network, though.
A operates at a higher frequency (5.8 ghz I believe) and thus would not be compatible with DS. Nor would a G router be compatible with anything operating on an A network.
So short and sweet: Get 802.11b/g network. G is backwards compatible, B only is fine, A will not work. This also means that wireless hotspots - Starbucks, Kinkos, etc - should all be able to be accessible through a DS (since most computers use B/G networks, public wireless networks are almost always B or G). I assume it won't be long that someone will offer internet browsers for use with the DS since you can get free access through those areas. Hell, I wonder if Nintendo can upgrade Pictochat to use a feature like that. Would be nice.
Also since I'm sure this will come up a LOT to Nintendo's tech support line (see comment below), if you are dropping connection a lot, see if you have cordless phones that operate at 2.4 ghz. They are known to cause problems and interrupt wireless connections (not always, but most of the time). And wireless routers only send signals so far, so make sure you are in range (usually 30 feet or less, depending on if you are going through walls, etc). It's not magically going to let you connect anywhere.
Bottomline: Get a B or G router. And for God's sake, make sure the thing is set up correct before you call into Nintendo to whine about crappy reception. Speak with Netgear/D-Link/etc first! It makes it easier for all of us!
(I do tech support for a laptop brand and get wireless questions all the time, pretty sure I'm correct here. Plus I'm trying to make you guys smarter consumers and users
My job would be a lot easier if people didn't call me and wonder why they can't connect to their wireless network in New York when they are in Florida.....)